The invention relates to a papermachine clothing of interwoven longitudinal and transverse threads in which the fabric weave has no axis of symmetry in the length direction and the fabric weave in a first region is mirror-symmetrical to that in a second region.
With the exception of a few types of fabric weaves, nearly all the weaves used in the manufacture of papermachine clothings have an asymmetrical structure, i.e. the binding points of the longitudinal wires with the transverse wires are lined in a diagonal direction. As a consequence of this asymmetrical structure, the fabric is more extensible in the diagonal direction than in a direction normal thereto so that the stress-strain behavior of the fabric is directional.
In all twill weaves, this diagonal structure, the so-called twill rib, is particularly pronounced. In satin weave fabrics, however, the binding points are uniformly distributed over the fabric area and do not contact each other. As a result, in each repeat pattern two, three, or more parallel diagonals are formed in lieu of the single diagonal in a twill weave. The appearance of the satin weave fabric is thus finer, and the marking left in the paper sheet is less striking, while the asymmetrical weave structure remains intact, and its effect is the stronger, the greater the asymmetry of the binding points of successive wires.
Under longitudinal stress, the tension in symmetrical weaves is distributed also symmetrically, i.e. the components of force directed to the right and to the left compensate each other. On the other hand, in asymmetrical weaves one component of force predominates. The fabric is thus subject to warping because the binding points directly adjacent or disposed close together render the deformability of the fabric directional. This effect is most pronounced in twill weaves. For this reason such fabrics are useful as papermachine fabrics in at best up to a 4-harness corkscrew twill, and then only on machines of very low specific power input, e.g. tissue machines. Even in these cases fabrics having a standard twill weave are subject to warping after a short time.
In this respect satin weaves are somewhat more favorable because the asymmetry is less pronounced. Thus, single-layer 5-harness satin-weave fabrics can be employed without any problems, and in double-layer fabrics, which are known to have a high degree of filling corresponding to 100-115% of the warp wires, fabrics made in 7 to 12-harness weave are employed. Yet, on high-speed papermachines operating at speeds of 900 m/minute and more regulation problems also arise with double-layer fabrics made in 7-harness weave because the fabrics migrate laterally in the direction of the diagonal rib.
Production is impaired even more by the lateral distortion of the fabric. Occasionally it happens that a fabric has become useless and must be cut out because in the tensioned region it is distorted too much in a lateral direction. While on the breast roll, at the point where the paper stock passes from the breast box to the fabric, the fabric is still perfectly centered, it may arrive at the other end of the fabric section with a lateral displacement of 7 to 10 cm. Since the paper web being formed migrates laterally together with the fabric, it may become removed from the working width of the dandy roller so that it is impossible to continue working with the fabric.
When the fabric diagonal is changed, e.g. when a right-hand diagonal instead of a left-hand diagonal is woven in the fabric, the fabric is distorted in the opposite direction with all the inherent problems for the papermaker. The construction of the papermaking machine in this case is so unfavorable in the upper fabric run that it is not possible to modify the machine design in order to counteract the fabric distortion. Only in the lower run can the fabric guide rolls be offset unilaterally in order to urge the fabric back to its desired middle position.
Distortion in the diagonal direction of a papermachine fabric with asymmetrical weave has been known. It has already been utilized to avoid grooving (groove-like erosion of the supporting surface) of the suction box covering. By altenately weaving the fabric with left-hand diagonal and right-hand diagonal ribs there is obtained a zig-zag extension of the warp wires to thereby avoid grooving of the suction box covering (German Auslegeschrift 1,710,373).
German Offenlegungsschrift 3,044,762 teaches a papermachine fabric in which in the marginal areas the weave alternates mirror-symmetrically from repeat to repeat. By this measure different long weft floats are attained in the marginal areas in order thereby delay the occurrence of tears in the length direction.
It is an object of the present invention to provide a papermachine clothing made of a fabric having a weave which does not have an axis of symmetry in the length direction but which also is not subject to lateral warping or distortion during use.